This invention relates to a process of producing gases which have a high calorific value and contain more than 50% methane by volume by a gasification of solid fuels, particularly coal, under a pressure of about 5-150 bars by a treatment with free oxygen-containing gas and water vapor and, if desired, additional gasifying agents to produce a water vapor-containing raw gas at a temperature of about 350.degree.-700.degree. C.
The process is derived from known processes of gasifying coal, including brown coal. A raw gas which can be economically converted to a high methane gas can be produced particularly by the pressure gasification of coal by a treatment with oxygen and/or air and, as further gasifying agents, water vapor and possibly carbon dioxide. A pressure gasification of coal is effected under pressures of 5-150 bars, preferably about 10-80 bars, and results in a water vapor-containing raw gas at a temperature of 350.degree.-700.degree. C. The pressure gasification of coal is known from numerous publications, such as U.S. Pat. Nos. 3,937,620, 3,902,872, 3,540,867 and 3,854,859, and German Published Specification DOS 2,201,859, and German Published Specification DOS 2,201,278.
Coal is normally gasified under pressure by a counterflow operation in which the fuel to be gasified and the gasifying agents are fed into the reaction chamber from opposite directions and move in said chamber in opposite directions. That operation has proved desirable because the sensible heat of the product gas is advantageously utilized to heat the fuel to the reaction temperature. In the reactor or gas producer, the fuel travels through several zones. The fuel is dried first and is then degasified in a dry distillation zone before the fuel enters the gasification zone, in which a major portion of the endothermic reactions are carried out. In the combustion zone the remaining fuel is finally reacted to a large extent with the free oxygen and an incombustible residual ash consisting of mineral constituents is left. The gasifying agent which flows into the reactor receives sensible heat from that ash; this is a special advantage from the aspect of heat economy. Experience has shown that the gasifying agents are suitably supplied at a metered rate which is selected so that the maximum combustion temperatures in the reactor are below the melting point of the ash.
In addition to water vapor, the raw gas produced by the pressure gasification of coal contains mainly hydrogen and carbon oxides as well as methane. Numerous further substances, such as condensible hydrocarbons, particularly tar having various boiling ranges, are present in smaller quantities. Whereas these are often considered as valuable constituents of coal, they are not always desirable. Unless they can be directly used for the production of energy, they must be fed to a further beneficiation stage, e.g., for hydrogenation. The processing of such substances is often problematic because they become available as a result of a gasification in quantities which are not sufficient for an economical utilization. They are also undesired because they become available together with the aqueous condensate formed from the gaseous constituents during the further processing of the raw gas. A considerable expenditure is required to purify this condensate, which contains not only hydrocarbons but, inter alia, also phenols, fatty acids, and ammonia.
It is an object of the invention to enable a processing of the raw gas and its conversion to a high-methane gas in a simpler manner and at lower costs. This is accomplished in that the water vapor-containing raw gas is reacted under a pressure of about 5-150 bars with free oxygen-containing gases in a succeeding reactor to produce an intermediate product gas, which contains mainly hydrogen, carbon oxides, and methane and which leaves the reactor at temperatures between about 600.degree. and 950.degree. C, and this intermediate product gas is cooled and freed from sulfur compounds. As the raw gas is converted to the intermediate product gas, the hydrocarbons contained in the raw gas as well as the disturbing phenols, fatty acids, and ammonia are converted mainly to hydrogen and carbon oxides by gasification and cracking and for this reason need not be separated from the raw gas and a further treatment is avoided. The reaction to produce the intermediate product gas is suitably effected under the same pressure as the production of the raw gas.
Dust fuels, particularly coal dust, or liquid hydrocarbons, particularly tar and/or tar oil, may preferably be gasified by a treatment with oxygen before or in the reactor for producing the intermediate product gas, and the gasification products may be fed to the reaction for producing the intermediate product gas. Exhaust gases and undesired by-products of other processes can also be processed by such thermal gasification treatment with oxygen. CO.sub.2 may be used as one of the gasifying agents in the production of the intermediate product gas.
The thermal gasification of the dust fuels, liquid hydrocarbons, exhaust gases or by-products by a treatment with oxygen results in reaction temperatures of about 900.degree.-1400.degree. C and in a production mainly of hydrogen and carbon monoxide, which subsequently deliver heat to the endothermic reactions carried out in the reactor for producing the intermediate product gas. The thermal gasification may be effected in a separate reactor or in the reactor for producing the intermediate product gas.
The dust fuels to be subjected to the thermal gasification have a particle size up to about 2 mm, preferably between about 0.03 mm and 0.3 mm. Liquid hydrocarbons to be subjected to the thermal gasification are first vaporized or formed into a fine spray. Exhaust gases which contain combustible constituents may be used as atomizing agents for dispersing the liquid hydrocarbons or dust fuels.
The reactor for producing the intermediate product gas may be designed in various ways. In the reactor, the starting materials in the form of dust and gas are suitably subjected to centrifugal forces or turbulent conditions of flow. This may be accomplished, e.g., in that the reactor contains internal fixtures or a bed of granular material having a particle size of about 3-80 mm, preferably about 5-30 mm. The granular material may consist of heat-resisting inert material, which serves primarily to agitate the gas and dust particles.
According to a further preferred feature of the process according to the invention, the reactor for producing the intermediate product gas contains catalytically acting substances, such as nickel, cobalt, chromium, or their oxides or sulfides. For this purpose, known catalysts are selected which accelerate the cracking of the gases and vapors to hydrogen and carbon oxides in the reactor for producing the intermediate product gas whereas a formation of carbon black is avoided. Supports for the catalysts may consist of Al.sub.2 O.sub.3, MgO or mixtures of these two substances as well as silicates of aluminum and/or magnesium. The catalyst support may also consist of aluminum spinel or magnesium spinel. To increase the extent of the reaction in the reactor for producing the intermediate product gas, the granular bed may be carried by a movable grate. Alternatively, the bed may consist of a fluidized bed.
Because the cracking reactions in the intermediate product gas reactor are endothermic reactions, care must be taken that sufficient energy is available for the reaction. To that end, the bulk material contained in the reactor may be periodically removed from the reactor and freed from combustible residues whereafter the bulk material is returned at an elevated temperature to the reactor. At least part of the energy required for the reaction may be supplied by high-frequency fields or electric resistance heating. In most cases, however, it will be possible to supply the required energy by a partial oxidation with the oxygen which is fed.